Project Summary Neuron loss and the reorganization of neural circuits in the superficial layers of entorhinal cortex are hallmarks of Alzheimer?s disease and temporal lobe epilepsy, and memory impairments are among the troubling symptoms of these diseases. Despite the knowledge that superficial entorhinal cell layers are selectively vulnerable in these diseases, the local connectivity of entorhinal circuits, how different functional cell types within these layers emerge, and how each cell type contributes to memory and spatial processing is only beginning to be revealed. The entorhinal cortex has extensive recurrent connectivity between its layers, and harbors many functional cell types such as grid cells, head-direction cells, border cells, context-selective cells, and other types of spatial and nonspatial cells. However, this functional diversity neither maps directly onto particular cell layers nor onto anatomically defined cell classes within layers. Each cell?s functional identity may therefore predominantly be determined by local microcircuits. The objective of this proposal is to examine whether functional cell identities in mEC, including grid cell firing and context-selective firing, emerge from circuit computations. We focus on local connectivity within the superficial layers and hypothesize that layer II pyramidal cells selectively contribute to rate coding in layer II stellate cells and that layer III inputs selectively contribute to spatial coding, including grid firing, in layer II stellate cells. This hypothesis will be tested in two specific aims. First, we will use viral tracing and patch clamp recordings with optical stimulation in entorhinal slices to determine the connectivity of mEC layer II pyramidal cells and layer III pyramidal cells and, for comparison, layer II stellate cells. Second, we will record from mEC cells in behaving mice while optogenetically stimulating or inhibiting entorhinal cell populations. In two separate subaims, we will examine (1) the effects of layer II pyramidal cell manipulations on the spatial and context-selective firing patterns of layer II stellate cells and layer III pyramidal cells and (2) the effects of layer III pyramidal cell manipulations on spatial and context- selective coding by layer II cells. Results from our aims will identify how local entorhinal circuits contribute to the generation of specialized entorhinal cell types, including grid cells and context-selective cells. This will not only fill gaps between theoretical models and experimental data, but also develop methods to selectively manipulate different functional cell types in mEC. Our results will therefore advance our understanding of the contribution of entorhinal cell types and cell layers to spatial and memory processing and thereby suggest strategies for restoring entorhinal circuit function and ameliorating the progression of neurodegenerative diseases.